1. Field of the Invention
The present invention relates to improvement of color toning of a heat sensitive recording material excellent in image storage stability.
2. Prior Art
In general, a heat sensitive recording material comprises a support and, provided thereon, a heat sensitive recording layer mainly composed of an electron-donating colorless dye precursor and an electron-accepting color developer and when this is heated by a thermal head, a thermal pen, laser beam, and the like, the colorless dye precursor and the color developer react instantaneously to produce a recorded image. Such is disclosed in Japanese Patent Kokoku Nos. 43-4160 and 45-14039. Such heat sensitive recording materials have the merits that recording can be performed by relatively simple devices, maintenance is easy and no noise is generated and are used in various fields such as recorders for meaurement, facsimile, printers, terminals for computers, labels and vending machines for tickets, etc.
Such heat sensitive recording materials which utilize electron-donating colorless dye precursor and electron-accepting color developer have various excellent properties that they have good appearance and are good to the touch and yield high coloring density and various hue, but they suffer from the problems that if colored portion (recorded image portion) contacts with plastic articles such as polyvinyl chloride, the portion disappears due to plastisizer or additives contained in the plastics, or if the portion contacts with chemicals contained in foods or cosmetics, it easily disappears or the portion is readily discolored upon exposure to light for a short period, namely, they are inferior in storage stability of record. Owing to these problems, they are limited in use and improvements on this point has been much demanded.
As heat sensitive recording materials which can provide recorded images of high storage stability by the reaction of two components upon heating, Japanese Patent Kokai Nos. 58-54085, 58-104959, 58-149388, 59-115887, and 59-115888 and U.S. Pat. No. 4,521,793 disclose heat sensitive recording materials using imino compound and isocyanate compound as the two components. These heat sensitive recording materials are superior in storage stability, but are inferior in heat responsivity and cannot give recorded image of sufficient density by a high speed printing apparatus. Furthermore, the resulting image is only of sepia color in hue.
As an approach to improve heat responsivity, the inventors reported a heat sensitive recording material containing three components of imino compound, isocyanate compound and blue color type fluoran compound in Japanese Patent Kokai No. 60-262686. This material is superior in storage stability of recorded image and heat responsivity, but is insufficient in toning effect and cannot form an image of color close to black. This has further defect that uncolored portion (background) is readily fogged.
The above heat sensitive recording material disclosed by the inventors is superior to conventional heat sensitive recording materials in image storage stability and heat responsivity, but has the problems that stability of uncolored portion is inferior (fogging in background) and hue of the image is not black color tone.